1. Field of the Invention
The present invention relates generally to a reflectometer tuning unit for a Magnetic Resonance Imaging (MRI) system, and more particularly pertains to a reflectometer tuning unit for a magnetic resonance imaging system, for adjusting and minimizing the standing wave voltage of an MRI receive coil unit.
Magnetic resonance imaging technology encompasses a wide spectrum of theoretical disciplines and practical equipment, the ultimate purpose of which is to achieve internal imaging of animate and inanimate objects through the monitoring of electromagnetic phenomenon, manifest at the nuclear level. In practice, MRI involves immersing the object under study in a high strength static magnetic field to align the magnetic spin of specific protons (primarily the protons of hydrogen atoms). In addition, a weak gradient magnetic field is applied to provide some spatial "tagging" on the object under study. At predetermined repetitive intervals, a strong pulse of radio frequency (RF) energy, at the MRI Larmor frequency, is transmitted by a set of coils whose field is orthogonal to the static electromagnetic field. The Larmor frequency of an MRI system is well known in the art, and is that characteristic frequency determined by the strength of the magnetic field and the species of element being excited. The Larmor frequency (F) is equal to the gyromagnetic constant (.gamma.) of the material being excited times the strength of the magnetic field (B). In a typical hydrogen proton MRI system, ##EQU1##
The perturbation caused by the pulse of RF energy causes a momentary upset in the magnetic spin of the nuclear species of the atoms within the object being imaged. Once the transmitted burst has subsided, these nuclear spins realign themselves to the influence of the original static magnetic field. The changing magnetic fields which result from this realignment induce various voltages in a receive coil which is placed in close proximity to the area of the object of interest prior to the imaging procedure. The signals picked up by the receive coil are then processed to ultimately produce high definition images of the object structures of interest.
The receive coil has evolved in design, from largely non-resonant broadband inductive pickup units, to coils which are designed to be sharply resonant at the Larmor frequency of the MRI system. Unlike broadband coils, resonant MRI receive coils possess the unique property of greatly improving the signal-to-noise ratio in the MRI signal processing chain, with concomitant improvement of MRI image quality and definition.
2. Discussion of the Prior Art
MRI is a well known technique wherein an object, animate or inanimate, which is placed in a spatially varying magnetic field is subjected to a pulse of Radio Frequency (RF) radiation, and the resulting nuclear magnetic resonance spectra are combined to give cross-sectional images of the object. The MRI technique is possible because the human body contains an abundance of hydrogen atoms, whose nuclei are protons, in its tissues, and these protons respond to electromagnetic manipulation, which is obviously essential in MRI. Generally, an MRI apparatus operates by the application of an RF excitation field in the presence of other magnetic fields, and by the subsequent sensing and analysis of the resulting nuclear magnetic resonance produced in the body.
Any nucleus which possesses a magnetic moment tends to align itself with the direction of the magnetic field in which it is located. Accordingly, when a substance such as human tissue is subjected to a static magnetic field, the individual magnetic moments of the protons in the tissue attempt to align with this polarizing magnetic field. However, the protons precess around the direction of the field at a characteristic angular frequency, known as the Larmor frequency, which is dependent on the strength of the magnetic field and the properties of the specific nuclear species. Once in the polarizing magnetic field, the alignments of the protons exist in one of two possible energy states which describe the spin angular momentum of the protons. Classically, the protons precess, that is, each proton's axis of rotation generally describes a cone and tends to turn at an angle relative to the direction of the applied polarizing magnetic field. The protons precess in a random order in terms of the phase of rotation. A net macroscopic magnetic moment is produced in the direction of the polarizing field, but the randomly orientated magnetic components in the perpendicular or transverse plane to the polarizing magnetic field cancel one another. If, however, the substance or tissue is subjected to an RF radiation pulse which is in the plane transverse to the polarizing magnetic field and which is at or near the Larmor frequency, the net aligned moment may be rotated or tipped into the transverse plane to produce a net transverse magnetic moment which is rotating or spinning in the transverse plane at or near the Larmor frequency. Essentially, the pulse of RF radiation is utilized to achieve resonance and produce a phase coherence such that the precessing protons are no longer random in phase, but rather at a single phase orientation. The degree to which the net magnetic moment is tipped, and hence the magnitude of the net transverse magnetic moment, depends primarily on the duration of time and the magnitude of the applied RF radiation signal.
The practical value of the above-described phenomenon resides in the signal which is emitted by the protons when the RF radiation pulse is terminated. Basically, a measurement is performed on the resonance signal emitted as feedback by the protons during the period when their magnetic moments tend to re-align themselves with the polarizing magnetic field. The measured signal is then processed in order to obtain therefrom cross-sectional images of the tissues or organs under examination. Essentially, as the protons are precessing and travelling back towards alignment within the polarizing magnetic field, they are "cutting" the plane of a receiving antenna which is part of the MRI device; accordingly, a current is induced in the receiving antenna in accordance with Faraday's Law. From this induced current signal, a map of the proton density of the tissue being imaged and its relaxation times, which is the time necessary for the protons to return to their unexcited condition, is generated. This feedback signal is processed and is ultimately transformed into a series of images of the tissue.
Various types of receiving antennas or coils have been designed for MRI applications. The most commonly utilized antenna is the standard sized whole body coil which is dimensioned to be disposed around the entire body of the patient to be imaged. Due to the standard sizing, a significant void or empty region may be defined between the coil and the portion of the patient to be imaged. As the imaged portion of the patient becomes a smaller fraction of the coil volume, the signal-to-noise ratio decreases, thereby degrading the image quality. In addition, the coil receives resonance signals from over a significantly larger area than the region of interest. This sensitivity to extraneous information degrades the spatial resolution. Accordingly, different size and shape receive coils are sometimes used in the prior art to minimize the above problems.